A luminaire or lighting fixture is a mechanical assembly of parts which must be capable of disassembly for lamp replacement, and in practice it is impossible to have it perfectly hermetically sealed. Therefore, it is accepted that even with hazardous location luminaires, any ambient gas will in time find its way into the luminaire. Lighting fixtures listed by Underwriters Laboratories (UL) as suitable for use in hazardous locations in which combustible gases or vapors are present (Class 1, Division 1), are required to have enclosures for the electrical components having sufficient strength to withstand the explosion pressure should there be an electrical or other malfunction that ignites the gases inside. Furthermore, as the momentary pressure from the explosion inside relieves itself to the outside, the ignited gases must be cooled sufficiently as they exit that explosive gases on the outside are not ignited, such quality being commonly referred to as nonflame propagation through the joints.
The light source proper within the luminaire may be either a filament or an inner arc tube in the case of a high intensity discharge (HID) lamp. In either case the lamp envelope operates at a very high temperature and usually is merely thin (1 mm thick) and relatively fragile glass. Accordingly, a globe is needed to surround and shield the lamp envelope from damage while permitting the light to pass out, and to keep ignitable gases away from the high temperature surface of the lamp envelope itself.
The globe is made of pressed glass generally at least 1/4 inch thick which has been tempered. In tempering, the glass globe is heated to a temperature adequate to relax internal stresses but permitting handling without deformation. The glass is then rapidly quenched by extracting heat from both surfaces at rates generating a symmetrical temperature profile across its thickness until the hottest point on the profile is below the effective solidification temperature. The result is a generally parabolic stress distribution across the thickness of the glass with compression at the surface balanced by tension in the interior.
Glass fractures when its tensile strength is exceeded and it is the outer surface that is vulnerable. With tempered glass, an external force such as a blow that would put an outer surface in tension must first neutralize the compressive prestress before any net tensile stress can develop at the surface, and thus breaking strength is enhanced. When tempered glass is broken, the strain energy reduces the glass into harmless small fragments more or less cubic in shape.
In globes for hazardous area luminaires as made up to the present, a thickened rim was provided which was generally at least twice as thick as the globe wall. The top of the rim was ground flat and engaged by the flat machined underside of a clamp ring to make a nonflame-propagating joint. The rim thickness was necessary in order to have a length of path across it sufficient for adequate cooling of exiting ignited gases, that is in order to avoid propagation of flame through the joint, as required by UL specifications. However the thick rim has been found to be a region in which over-tempering tends to occur and a source of inherent weakness, as disclosed in copending application Ser. No. 564,117, filed of even date herewith by Marcus P. Hogue, entitled Tempered Glass Globe, and assigned to the same assignee as the present invention. The Hogue invention eliminates the thickened rim and provides a tempered glass globe of substantially constant wall thickness throughout. This makes possible a more uniform cooling rate during the tempering quench cycle whereby overtempering and excessive tensile stresses within the glass are avoided.
With the Hogue constant wall thickness globe, the rim at the top of the globe is no different in wall thickness and is outwardly flared at an angle between 15.degree. and 45.degree.. The globe is supported by nesting the section of cone formed by the flared rim in a mating conically apertured portion formed in a globe support ring. The globe is held down snugly in the globe ring by clamping means such as bolts and washers or spring clips provided around the periphery of the ring and engaging the top of the rim.
For a hazardous location lighting fixture, Underwriters Laboratories specification 844 defines the standard under which a joint must be qualified by test in order to be deemed nonflame propagating. It specifies the minimum length of the joint and the maximum clearance between surfaces at the joint, that is the maximum gap. The requirement in respect of joint or gap dimensions is related to the internal volume of the fixture and is expressed as a linear relationship between gap clearance amd length of gap. For examle, if the length of the joint is 1.125", a gap of 0.004" is acceptable; but if the joint is only 0.625" long, then the gap is limited to 0.0015". These constraints must be observed between the flared rim of the Hogue globe and the mating conically apertured portion of the support ring. But it is not yet technically feasible to press and temper a glass globe so that the rim is exactly true. It has been necessary to resort to grinding the flared glass rim in order to bring it within the specification requirements. Such grinding is expensive and, depending upon the extent needed, may amount to a substantial fraction of the cost of pressing and tempering the glass. Also grinding inevitably reduces the strength of the globe.